1. Field of the Invention
The present invention generally relates to a light modulator for modulating a laser beam and, more particularly, to a high speed light modulator of a kind used in a high speed optical fiber communication system. The present invention also relates to a method of manufacturing such light modulator.
2. Description of the Prior Art
In the high speed optical fiber communication system, a considerable amount of data are transmitted by the use of semiconductor laser beams and optical fibers. In order to cope with this feature, the semiconductor laser beams are required to be modulated at a high speed. With the conventional direct modulation system in which the electric current injected to a single-mode semiconductor laser is modulated to provide the modulated output laser beam, change in wavelength resulting from change in density of injected carriers (i.e., wavelength chirping) is so substantial that the conventional direct modulation system cannot be used in high-speed modulation of 10 Gbps or higher.
In view of the foregoing, as an alternative to the direct modulation system, the external modulation system has come to be the cynosure of those concerned, in which a light modulator having a low chirping and disposed externally of a semiconductor laser is utilized to modulate the laser beam while the current injected to the semiconductor laser is fixed. The combined modulator and laser assembly in which a light modulator, a single-mode semiconductor laser and an isolator separating the light modulator and the semiconductor laser from each other are integrated together on a single chip is shown by 60 in FIG. 7. Since no circuit is required between the modulator and the laser, the combined modulator and laser assembly 60 shown therein has a high practical utility and is extremely important as a key device for optical fiber communication of a large amount of data.
The light modulator will now be described. As shown in FIG. 8A, the light modulator 70 includes an InP semiconductor substrate 52 on which a semiconductor mesa layer 56 of a predetermined width containing a light absorption layer 51 and a semiconductor bonding pad layer 55 are formed. The laser beam inputted to the light modulator 70 is modulated by the light absorption layer 51. More specifically, by applying a voltage to the bonding pad electrode 55a, an electric field is applied from an electrode 54, covering the semiconductor mesa layer 56, to the light absorbing layer 51, and by shifting the absorption wavelength of the light absorbing layer 51, the input laser beam is modulated.
As shown in FIG. 8B, a groove 57 is formed between the semiconductor mesa layer 56 and the semiconductor bonding pad layer 55 for separating the semiconductor layers 55 and 56 from other semiconductor layers. The semiconductor mesa layer 56, the semiconductor bonding pad layer 55 and the groove 57 has their respective surfaces covered by a continuous insulating film 53. The bonding pad electrode 55a and the electrode 54 are formed by a metallic film continuously covering the insulating film 53 while the electrode 54 is held in ohmic contact with the semiconductor mesa layer 56 through an opening defined in the insulating film 53.
The conventional method of manufacturing the conventional light modulator is shown in FIGS. 9A to 9C. Referring first to FIG. 9A, a predetermined crystalline layer is epitaxially grown on the InP substrate 52 to form the semiconductor mesa layer 56 of the predetermined width including the light absorption layer 51, the groove 57 and the semiconductor bonding pad layer 55. Then, as shown in FIG. 9B, the insulating film 53 of SiO2 having a film thickness of about 4000 xc3x85 is formed so as to cover the entire surface of the InP substrate 52. After a window for the ohmic contact has been formed in an upper surface of the semiconductor mesa layer 54 including the light absorption layer 51, the metallic film is formed at a predetermined location as shown in FIG. 9C to complete the bonding pad electrode 55a and the electrode 54.
In order for the light modulator to be used for high-speed modulation, it is necessary to reduce the static capacitance (hereinafter referred to as a xe2x80x9cparasitic static capacitancexe2x80x9d) formed between surface electrodes (the bonding pad electrode 55a and the electrode 54) and a rear surface electrode. The parasitic static capacitance of the light modulator is expressed by the sum of the parasitic static capacitance of the mesa layer 56 plus the parasitic static capacitance of the bonding pad layer 55. In order to reduce the parasitic static capacitance of the light modulator, attempts have currently been made to minimize the surface area of each of the mesa layer 54 and the bonding pad layer 55 by forming the groove 57 therebetween.
It has, however, been found that considering the chirping of light that is propagated by the light absorption layer 51, the width of the mesa layer 56 can only be reduced to a certain limited dimension. Also, considering the bonding surface area of the bonding wire, the size of the bonding pad layer 55 is limited to about 50xc3x9750 xcexcm. Thus, the approach to reduce the surface area of the mesa layer 56 and the bonding pad layer 55 in an attempt to reduce the parasitic static capacitance is limited and, therefore, a sufficiently high-speed modulation characteristic has been difficult to accomplish.
The present invention has therefore been developed in view of the foregoing problems and is intended to provide an improved light modulator capable of accomplishing a high-speed light modulation in which the parasitic static capacitance is reduced and also to provide an improved method of manufacturing such light modulator.
The light modulator of the present invention is such that the parasitic static capacitance of the bonding pad section has been reduced to substantially eliminate the above discussed problems, and is therefore effective to achieve the high-speed modulation. More specifically, the light modulator of the present invention includes a semiconductor substrate having first and second surfaces opposite to each other with a grounding conductor formed on the second surface thereof. A mesa section of a predetermined width laminated with a semiconductor layer including a light absorption layer and a bonding pad forming section adjacent the mesa section are formed on the semiconductor substrate. An insulating layer continuing from the mesa section to the bonding pad section is formed with an opening defined in a portion of the insulating film above the mesa section, and an electrode contacting an upper surface of the mesa section through the opening and extending to the bonding pad forming section is formed over the insulating layer. Accordance with the present invention, the light modulator is featured in that a portion of the insulating layer the bonding pad forming section has a thickness greater than that of the remaining portion of the insulating layer to reduce the parasitic static capacitance of the bonding pad section.
The portion of the insulating layer immediately above the bonding pad forming section comprises a multi layered structure containing at least insulating films laminated one above other. The remaining portion of the insulating layer comprises a single or multi layered structure containing a insulating films, in which a number of the insulating film is less than that of the bonding pad forming section.
The insulating films are two insulating films, one of the two insulating films is made of SiO2 and the other is made of SiN.
The upper-layer insulating film of remaining portion of the insulating layer is same as the 2nd upper-layer insulating film of the bonding pad forming section.
The first method of manufacturing the light modulator according to the present invention is such that the parasitic static capacitance of the bonding pad section has been reduced to substantially eliminate the above discussed problems. More specifically, this first method is utilized to manufacture the light modulator which includes a semiconductor substrate having first and second surfaces opposite to each other with a grounding conductor formed on the second surface thereof, which substrate is formed with a mesa section of a predetermined width, laminated with a semiconductor layer including a light absorption layer, and a bonding pad forming section adjacent the mesa section, an insulating layer continuing from the mesa section to the bonding pad section and formed with an opening defined in a portion of the insulating layer above the mesa section, and a one-piece electrode formed over the insulating film and contacting an upper surface of the mesa section through the opening, the one-piece electrode forming a bonding pad electrode. This first method is featured in that it comprises forming a primary insulating film continuing from the mesa section to the bonding pad forming section, forming a mask so as to cover a portion of the primary insulating film formed above the bonding pad forming section, etching the primary insulating film to remove another portion of the primary insulating film other than that portion of the primary insulating film above the bonding pad forming section, removing the mask forming a secondary insulating film continuing from that portion of the primary insulating film above the bonding pad forming section and the mesa section and completing the insulating whereby that portion of the insulating layer above the bonding pad forming section has a thickness greater than that of the remaining portion of the insulating layer to reduce the parasitic static capacitance of the bonding pad section.
The second method of manufacturing the light modulator according to the present invention is such that the parasitic static capacitance of the bonding pad section has been reduced to substantially eliminate the above discussed problems. More specifically, this second method is utilized to manufacture the light modulator which includes a semiconductor substrate having first and second surfaces opposite to each other with a grounding conductor formed on the second surface thereof, which substrate is formed with a mesa section of a predetermined width, laminated with a semiconductor layer including a light absorption layer, and a bonding pad forming section adjacent the mesa section, an insulating layer continuing from the mesa section to the bonding pad section and formed with an opening defined in a portion of the insulating layer above the mesa section, and a one-piece electrode formed over the insulating film and contacting an upper surface of the mesa section through the opening, the one-piece electrode forming a bonding pad electrode. This second method is featured in that it comprises forming a primary insulating film continuing from the mesa section to the bonding pad forming section, forming a mask so as to cover a portion of the primary insulating film other than a portion of the primary insulating film that is formed above the bonding pad forming section, forming a secondary insulating film over the mask and that portion of the primary insulating film above the bonding pad forming section, removing the mask to allow that portion of the secondary insulating film above the bonding pad section to continue to that portion of the primary insulating film above the mesa section to thereby complete the insulating layer so that that portion of the insulating layer above the bonding pad forming section has a thickness greater than that of the remaining portion of the insulating layer to reduce the parasitic static capacitance of the bonding pad section.
The third method of manufacturing the light modulator according to the present invention is such that the parasitic static capacitance of the bonding pad section has been reduced to substantially eliminate the above discussed problems. More specifically, this third method is utilized to manufacture the light modulator which includes a semiconductor substrate having first and second surfaces opposite to each other with a grounding conductor formed on the second surface thereof, which substrate is formed with a mesa section of a predetermined width, laminated with a semiconductor layer including a light absorption layer, and a bonding pad forming section adjacent the mesa section, an insulating layer continuing from the mesa section to the bonding pad section and formed with an opening defined in a portion of the insulating layer above the mesa section, and a one-piece electrode formed over the insulating layer and contacting an upper surface of the mesa section through the opening, the one-piece electrode forming a bonding pad electrode. This third method is featured in that it comprises a primary insulating film forming step of forming a primary insulating film continuing from the mesa section to the bonding pad forming section, forming a mask so as to cover a portion of the primary insulating film above the bonding pad forming section, etching another portion of the primary insulating film other than that portion of the primary insulating film above the bonding pad forming section to a predetermined thickness, removing the mask so as to leave the insulating film having a thick film portion above the bonding pad forming section and a thin film portion above the mesa section, the thick and thin film portion being continued together, and completing the insulating layer whereby that portion of the insulating layer above the bonding pad forming section has a thickness greater than the remaining portion of the insulating layer to thereby reduce the parasitic static capacitance of the bonding pad section.
The primary insulating film forming step of the third method of the present invention discussed above may include forming an under-layer insulating film continuing from the mesa section to the bonding pad forming section, forming over the under-layer insulating film an intermediate-layer insulating film of a material different from that of the under-layer insulating film, and forming over the intermediate-layer insulating film an upper-layer insulating film of the same material as that of the under-layer insulating film. In such case, the etching step may be carried out for selectively etching only a portion the upper-layer other than that formed above the bonding pad section, and completing the insulating layer.
Preferably, in any of the first to third method, the insulating film is made of SiO2 or SiN.
In the practice of the third method, one of the primary insulating film and the secondary insulating film is preferably made of SiO2 while the other thereof is preferably made of SiN.
Also, in the practice of any one of the first to third methods of the present invention, the insulating film is preferably formed by the use of a CVD technique, a sputtering technique or a vacuum evaporation technique.